<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title></title>
    <style>
       .box{
           background: #000;
          display: inline-block;
          border: 10px solid blue;
          padding: 40px;
          margin: 40px;
       } 
       .child{
            background-color: red;
            width: 100px;
            height: 100px;
       }
          </style>
</head>
<body>
    <div class="box">
        <div class="child">
             a
        </div>
    </div>
</body>
</html>
